1. Field of the Invention
This invention relates generally to semiconductor processing, and more particularly to semiconductor chip solder bump pads and methods of making the same.
2. Description of the Related Art
Flip-chip mounting schemes have been used for decades to mount semiconductor chips to circuit boards, such as semiconductor chip package substrates. In many conventional flip-chip variants, a plurality of solder joints are established between input/output (I/O) sites of a semiconductor chip and corresponding I/O sites of a circuit board. In one conventional process, a solder bump is metallurgically bonded to a given I/O site or pad of the semiconductor chip and a so-called pre-solder is metallurgically bonded to a corresponding I/O site of the circuit board. Thereafter the solder bump and the pre-solder are brought into proximity and subjected to a heating process that reflows one or both of the solder bump and the pre-solder to establish the requisite solder joint.
In one conventional process, the connection of the solder bump to a particular I/O site of a semiconductor chip entails forming an opening in a top-level dielectric film of a semiconductor chip proximate the I/O site and thereafter depositing metal to establish an underbump metallization (UBM) structure. The solder bump is then metallurgically bonded to the UBM structure by reflow. This conventional UBM structure includes a base, a sidewall and an upper flange that is positioned on the dielectric film.
Flip-chip solder joints may be subjected to mechanical stresses from a variety of sources, such as coefficient of thermal expansion mismatches, ductility differences and circuit board warping. Such stresses can subject the just described conventional UBM structure to bending moments. The effect is somewhat directional in that the stresses tend to be greatest nearer the die edges and corners and fall off with increasing proximity to the die center. The bending moments associated with this so-called edge effect can impose stresses on the dielectric film beneath the UBM structure that, if large enough, can produce fracture.
For a variety of reasons, designers have begun to turn to lead-free solders for solder joint fabrication. Bumps composed from such solders may produce higher stresses than comparably sized lead-based bumps. To compensate for these higher stresses, a conventional design incorporates a terminal pad between the underbump metallization and the underlying chip bump pad. The terminal pad has a larger footprint than the overlying underbump metallization and the underlying chip pad to provide stress protection for the passivation layer. If the chip includes active traces proximate the chip pad, the terminal pad may overlap such traces and give rise to parasitics.
The present invention is directed to overcoming or reducing the effects of one or more of the foregoing disadvantages.